vcs-svn: handle_node: use repo_read_path

vcs-svn: handle_node: use repo_read_path

svn-fe processes each commit in two stages: first decide on the
correct content for all paths and export the relevant blobs, then
export a commit with the result.

But we can keep less state and simplify svn-fe a great deal by
exporting the commit in one step: use 'inline' blobs for each path and
remember nothing.  This way, the repo_tree structure could be
eliminated, and we would get support for incremental imports 'for
free'.

Reorganize handle_node along these lines.  This is just a code
cleanup; the changes in repo_tree and handle_revision will come later.

[db: backported to apply without text delta support]

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>